Set error code when no source is specified
For failing with errors in CI/CD scripts, it would be nice if cmake
returned non-0 exit code when source directory is missing.
$ cmake
Usage
cmake [options] <path-to-source>
cmake [options] <path-to-existing-build>
Specify a source directory to (re-)generate a build system for it in the
current working directory. Specify an existing build directory to
re-generate its build system.
Run 'cmake --help' for more information.
$ echo $?
0